A system for Photodynamic Therapy (PDT) which provides, in addition to the therapeutic light dose, a method of real-time intra-operative dosimetry will be assembled and demonstrated. In PDT, a parenterally administered photoactive dye (dihematoporphyrin ether, DHE) localizes in malignant tissue. Subsequent irradiation of the DHE bearing tissue with 630nm light produces, in combination with oxygen, the cytotoxic reaction product, singlet oxygen. By monitoring the real-time- resolved singlet oxygen fluorescence at 1.27 micromolar, a signal proportional to the singlet oxygen production rate will result, thereby providing dosimetry information. The system to be demonstrated employs a CW-pumped Q- switched, frequency-doubled YAG laser driving a tunable dye laser emitting 70 nanosecond, 630nm wavelength pulses at a 10KHz rate. In addition to providing real-time dosimetry information, the system also allows for the study of a possible increase in the therapeutic effect due to the generation of the cytotoxic agent singlet oxygen in "bursts" since the reciprocity of the PDT light dose appears not to have been investigated thus far. Finally, the inherent wavelength agility of a tunable dye laser makes this system useful with new photoactive dyes operating beyond 630nm.